Latch-up prevention for memory cells

ABSTRACT

An SRAM memory cell is provided having a pair of cross-coupled CMOS inverters. The sources of the pull-up transistors forming each of the CMOS inverters are coupled to V CC  through parasitic resistance of the substrate in which each is formed. The source of the p-type pull-up transistor is therefore always at a potential less than or equal to the potential of the N-well such that the emitter-base junction of the parasitic PNP transistor cannot become forward biased and latch-up cannot occur.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 09/740,174 filedDec. 18, 2000, now U.S. Pat. No. 6,767,784 issued Jul. 27, 2004 entitled“LATCH-UP PREVENTION FOR MEMORY CELLS”, which is a division ofapplication Ser. No. 09/368,710, filed Aug. 5, 1999, now U.S. Pat. No.6,376,297, issued Apr. 23, 2002 and entitled “LATCH-UP PREVENTION FORMEMORY CELLS”, which is a division of application Ser. No. 09/045,465filed Mar. 20, 1998, now U.S. Pat. No. 6,005,797, issued Dec. 21, 1999and entitled “LATCH-UP PREVENTION FOR MEMORY CELLS”.

This application is also related to U.S. patent application Ser. No.09/620,055, filed Jul. 20, 2000, now U.S. Pat. No. 6,642,588 B1, issuedNov. 4, 2003 and entitled “LATCH-UP PREVENTION FOR MEMORY CELLS”, whichapplication is a continuation of U.S. patent application Ser. No.09/368,710, filed Aug. 5, 1999, now U.S. Pat. No. 6,376,297, issued Apr.23, 2002 and entitled “LATCH-UP PREVENTION FOR MEMORY CELLS”.

BACKGROUND OF THE INVENTION

The present invention relates in general to a static random accessmemory (SRAM), and, more particularly, to an SRAM having improvedlatch-up characteristics.

RAM chips are well known in the art. An SRAM chip is conventionallystructured in rows and columns of individual SRAM cells. A prior art sixtransistor CMOS SRAM cell 1 is shown schematically in FIG. 1. The SRAMcell 1 includes two n-type access transistors 5, 6, two p-type pull-uptransistors 7, 8 acting as load devices, and two n-type pull-downtransistors 9, 10, with the pull-transistors 7,8 and pull-downtransistors 9, 10 forming two CMOS inverters. The SRAM cell 1 has twostates: logic state “0” and logic state “1”. By convention, if logicstate “0” is designated by node A having a high voltage and node Bhaving a low voltage, then logic state “1” has the opposite storedvoltages, i.e., node A having a low voltage and node B having a highvoltage.

In logic state “0” the high voltage on node A turns on the pull-downtransistor 9 and turns off the pull-up transistor 7, whereas the lowvoltage on node B turns off the pull-down transistor 10 and turns on thepull-up transistor 8. Because the pull-down transistor 9 is on and thepull-up transistor 7 is off, current flows through the pull-downtransistor 9 to a voltage supply V_(SS) (ground), thereby maintaining alow voltage on node B. Because the pull-up transistor 8 is turned on andthe pull-down transistor 10 is turned off, current flows from a voltagesupply V_(CC) through the pull-up transistor 8, thereby maintaining ahigh voltage on node A.

To change the state of the SRAM cell 1 from a logic “0” to a logic “1”,a column line 3 and a column line complement 2 are provided with a lowand a high voltage, respectively. Then, the access transistors 5 and 6are turned on by a high voltage on a row line 4, thereby providing thelow voltage on the column line 3 to node A and the high voltage on thecolumn line complement 2 to node B. Accordingly, the pull-downtransistor 9 is turned off and the pull-up transistor 7 is turned on bythe low voltage on node A and the pull-down transistor 10 is turned onand the pull-up transistor 8 is turned off by the high voltage on nodeB, thereby switching the state of the circuit from logic “0” to logic“1”. Following the switching of the state of the SRAM cell 1, the accesstransistors 5 and 6 are turned off (by applying a low voltage on rowline 4). The SRAM cell 1 maintains its new logic state in a manneranalogous to that described above.

FIGS. 2A and 2B are a schematic diagram and cross-section, respectively,of one of the CMOS inverters of FIG. 1 illustrating parasitictransistors and resistors of the inverter. As shown in FIG. 2B, thepull-down transistor 9 is formed within a P-type substrate 12 while thepull-up transistor 7 is formed within an N-well 14. The N-well 14 isformed within the P-type substrate 12. The N-well 14 includes parasiticresistance denoted by the resistor 16 and the P-type substrate includesparasitic resistance denoted by the resistor 18. The configuration ofthe pull-down transistor 9 and the pull-up transistor 7 results in theexistence of a PNP parasitic bipolar transistor 20 and an NPN parasiticbipolar transistor 22.

With the tight layout spacings that exist in a typical memory array,leakage currents from the N-well 14 and the P-type substrate 12 arepossible. These leakage currents produce a voltage drop across theparasitic resistor 16. If the voltage drop becomes sufficiently large,it can result in the parasitic PNP transistor 20 turning on andconducting current from the P+ region forming its emitter to the P-typesubstrate 12 that forms its collector. The P-type substrate 12 alsoforms the base terminal of the parasitic NPN transistor 22 and oneterminal of the parasitic resistor 18. The other terminal of theparasitic resistor 18 is the substrate tie-down represented by V_(BB).The current flowing through the resistor 18 produces a voltage rise atthe point of injection. If this voltage rise becomes sufficiently large,it can result in the NPN transistor 22 turning on causing additionalcurrent to be drawn out the N-well 14 as collector current for the NPNtransistor 22. This additional current reinforces the original leakagefrom the N-well 14 turning the PNP transistor 20 on even harderproviding added base current for the NPN transistor 22. This feedbackloop can result in a latch-up problem within the memory array containingthe SRAM cell.

Accordingly, there is a need for an improved SRAM memory cell that isnot prone to latch-up.

SUMMARY OF THE INVENTION

The present invention meets this need by providing an SRAM memory cellin which the source of the p-type pull-up transistor is coupled toV_(CC) through the parasitic resistance of the N-well in which it isformed. The source of the p-type pull-up transistor is therefore alwaysat a potential less than or equal to the potential of the N-well suchthat the emitter-base junction of the parasitic PNP transistor cannotbecome forward biased and latch-up cannot occur.

According to a first aspect of the present invention, an SRAM memorycell is provided comprising a first pull-up transistor having a firstsubstrate, a first source, a first gate coupled to a first node, and afirst drain coupled to a second node. The first source is coupled to afirst voltage input through parasitic resistance of the first substrate.A first pull-down transistor is provided having a second drain coupledto the second node, a second gate coupled to the first node, and asecond source coupled to a second voltage source. An input line iscoupled to the first node for providing a signal to the memory cell tochange the memory cell from a first logic state to a second logic state.The input line may comprise an access transistor having one terminalcoupled to the first node, another terminal coupled to a column line,and an access gate coupled to a row line.

According to another aspect of the present invention, an SRAM memorycell is provided comprising a first pull-up transistor, a first pulldown transistor, a second pull-up transistor, a second pull-downtransistor, and an input line. The first pull-up transistor includes afirst substrate, a first source, a first gate coupled to a first node,and a first drain coupled to a second node. The first pull-downtransistor includes a second drain coupled to the second node, a secondgate coupled to the first node, and a second source coupled to a secondvoltage input. The second pull-up transistor includes a secondsubstrate, a third source, a third gate coupled to the second node, anda third drain coupled to the first node. The second pull-down transistorincludes a fourth drain coupled to the first node, a fourth gate coupledto the second node, and a fourth source coupled to the second voltageinput. The first source is coupled to a first voltage input throughparasitic resistance of the first substrate while the third source iscoupled to the first voltage input through parasitic resistance of thesecond substrate. The input line is coupled to the first and secondnodes for providing a signal to the memory cell to change the memorycell from a first logic state to a second logic state. Preferably, thefirst substrate and the second substrate are portions of a singlesubstrate.

According to yet another aspect of the present invention, an SRAM memorycell is provided comprising a substrate assembly having at least onesemiconductor layer, and a first semiconductor structure formed withinthe at least one semiconductor layer with the first semiconductorstructure being coupled to a first voltage input. A first pull-uptransistor is formed in the first semiconductor structure and comprisesa first gate, a first source and a first drain. The first source iscoupled to the first semiconductor structure such that the first sourceis coupled to the first voltage input through parasitic resistance ofthe semiconductor structure. A first pull-down transistor is formed inthe at least one semiconductor layer and comprises a second gate coupledto the first gate, a second source coupled to a second voltage input,and a second drain coupled to the first drain.

The memory cell may further comprise an access transistor formed in theat least one semiconductor layer having one terminal coupled to thefirst and second drains, another terminal coupled to a column line andan access gate coupled to a row line. Preferably, the at least onesemiconductor layer comprises P-type semiconductor material and thesemiconductor structure comprises an N-type well

According to a further aspect of the present invention, an SRAM memorycell comprises a substrate assembly having at least one semiconductorlayer, and a first semiconductor structure and a second semiconductorstructure formed within the at least one semiconductor layer with thefirst and second semiconductor structures being coupled to a firstvoltage input. A first pull-up transistor is formed in the firstsemiconductor structure and comprises a first gate, a first source and afirst drain with the first source being coupled to the firstsemiconductor structure such that the first source is coupled to thefirst voltage input through parasitic resistance of the firstsemiconductor structure. A first pull-down transistor is formed in theat least one semiconductor layer and comprises a second gate coupled tothe first gate, a second source coupled to a second voltage input, and asecond drain coupled to the first drain. A second pull-up transistor isformed in the second semiconductor structure and comprises a third gate,a third source and a third drain with the third source being coupled tothe second semiconductor structure such that the third source is coupledto the first voltage input through parasitic resistance of the secondsemiconductor structure. A second pull-down transistor is formed in theat least one semiconductor layer and comprises a fourth gate coupled tothe third gate, a fourth source coupled to the second voltage input, anda fourth drain coupled to the third drain.

Preferably, the at least one semiconductor layer comprises P-typesemiconductor material while the first and second semiconductorstructures each comprise an N-type well. The first semiconductorstructure and the second semiconductor structure may form portions of asingle semiconductor structure. The memory cell may further comprise afirst access transistor formed in the at least one semiconductor layerhaving a first terminal coupled to the first and second drains, a secondterminal coupled to a first column line and a first access gate coupledto a row line, and a second access transistor formed in the at least onesemiconductor layer having a third terminal coupled to the third andfourth drains, a fourth terminal coupled to a second column line and asecond access gate coupled to the row line.

According to a still further aspect of the present invention an SRAMmemory array is provided comprises a plurality of memory cells arrangedin rows and columns. Each of the memory cells comprise a first pull-uptransistor, a first pull-down transistor, a second pull-up transistor, asecond pull-down transistor, a first access transistor and a secondaccess transistor. The first pull-up transistor includes a firstsubstrate, a first source, a first gate coupled to a first node, and afirst drain coupled to a second node with the first source being coupledto a first voltage input through parasitic resistance of the firstsubstrate. The first pull-down transistor includes a second draincoupled to the second node, a second gate coupled to the first node, anda second source coupled to a second voltage input. The second pull-uptransistor includes a second substrate, a third source, a third gatecoupled to the second node, and a third drain coupled to the first nodewith the third source being coupled to the first voltage input throughparasitic resistance of the second substrate. The second pull-downtransistor includes a fourth drain coupled to the first node, a fourthgate coupled to the second node, and a fourth source coupled to thesecond voltage input. The first access transistor includes a firstterminal coupled to the first and second drains, a second terminalcoupled to a first column line and a first access gate coupled to a rowline. The second access transistor includes a third terminal coupled tothe third and fourth drains, a fourth terminal coupled to a secondcolumn line and a second access gate coupled to the row line. The memoryarray also includes a memory decoder coupled to the plurality of memorycells for accessing each of the plurality of memory cells via therespective ones of a plurality of the row lines and respective ones of aplurality of the first and second column lines.

Preferably, the first substrate and the second substrate form portionsof a single substrate. The memory array may comprise a plurality of therows with each of the first and second pull-up transistors making upeach of the rows of memory cells sharing a common substrate.

According to yet another aspect of the present invention, an SRAM memoryarray is provided comprising a plurality of SRAM memory cells arrangedin rows and columns and formed on a substrate assembly comprises atleast one semiconductor layer. Each of the plurality of memory cellscomprise a first semiconductor structure and a second semiconductorstructure formed within the at least one semiconductor layer with thefirst and second semiconductor structures is coupled to a first voltageinput. A first pull-up transistor is formed in the first semiconductorstructure and comprises a first gate, a first source and a first drainwith the first source being coupled to the first semiconductor structuresuch that the first source is coupled to the first voltage input throughparasitic resistance of the first semiconductor structure. A firstpull-down transistor is formed in the at least one semiconductor layerand comprises a second gate coupled to the first gate, a second sourcecoupled to a second voltage input, and a second drain coupled to thefirst drain. A second pull-up transistor is formed in the secondsemiconductor structure and comprises a third gate, a third source and athird drain with the third source being coupled to the secondsemiconductor structure such that the third source is coupled to thefirst voltage input through parasitic resistance of the secondsemiconductor structure. A second pull-down transistor is formed in theat least one semiconductor layer and comprises a fourth gate coupled tothe third gate, a fourth source coupled to the second voltage input, anda fourth drain coupled to the third drain. A first access transistor isformed in the at least one semiconductor layer and includes a firstterminal coupled to the first and second drains, a second terminalcoupled to a first column line and a first access gate coupled to a rowline. A second access transistor is formed in the at least onesemiconductor layer and includes a third terminal coupled to the thirdand fourth drains, a fourth terminal coupled to a second column line anda first access gate coupled to a row line. The memory array includes amemory decoder coupled to the plurality of memory cells for accessingeach of the plurality of memory cells via respective ones of a pluralityof the row lines and respective ones of a plurality of the first andsecond column lines.

Preferably, the first substrate and the second substrate form portionsof a single substrate. The at least one semiconductor layer may compriseP-type semiconductor material while the first semiconductor structuremay comprise an N-type well. The memory array may comprise a pluralityof the rows with each of the first and second pull-up transistors makingup each of the rows of memory cells sharing the N-type well.

According to yet another aspect of the present invention, a computersystem is provided comprising an SRAM memory array and a microprocessor.The memory array comprises a plurality of memory cells arranged in rowsand columns with each of the memory cells comprising a first pull-uptransistor, a first pull-down transistor, a second pull-up transistor, asecond pull-down transistor, a first access transistor and a secondaccess transistor. The first pull-up transistor includes a firstsubstrate, a first source, a first gate coupled to a first node, and afirst drain coupled to a second node with the first source being coupledto a first voltage input through parasitic resistance of the firstsubstrate. The first pull-down transistor includes a second draincoupled to the second node, a second gate coupled to the first node, anda second source coupled to a second voltage input. The second pull-uptransistor includes a second substrate, a third source, a third gatecoupled to the second node, and a third drain coupled to the first nodewith the third source being coupled to the first voltage input throughparasitic resistance of the second substrate. The second pull-downtransistor includes a fourth drain coupled to the first node, a fourthgate coupled to the second node, and a fourth source coupled to thesecond voltage input. The first access transistor includes a firstterminal coupled to the first and second drains, a second terminalcoupled to a first column line and a first access gate coupled to a rowline. The second access transistor includes a third terminal coupled tothe third and fourth drains, a fourth terminal coupled to a secondcolumn line and a second access gate coupled to the row line. The memoryarray also includes a memory decoder coupled to the plurality of memorycells for accessing each of the plurality of memory cells via therespective ones of a plurality of the row lines and respective ones of aplurality of the first and second column lines. The microprocessorcommunicates with each of the plurality of memory cells via the memorydecoder.

According to a further aspect of the present invention, a computersystem is provided comprising a SRAM memory array and a microprocessor.The memory array comprises a plurality of memory cells arranged in rowsand columns and formed on a substrate assembly comprises at least onesemiconductor layer. Each of the plurality of memory cells comprise afirst semiconductor structure and a second semiconductor structureformed within the at least one semiconductor layer with the first andsecond semiconductor structures is coupled to a first voltage input. Afirst pull-up transistor is formed in the first semiconductor structureand comprises a first gate, a first source and a first drain with thefirst source being coupled to the first semiconductor structure suchthat the first source is coupled to the first voltage input throughparasitic resistance of the first semiconductor structure. A firstpull-down transistor is formed in the at least one semiconductor layerand comprises a second gate coupled to the first gate, a second sourcecoupled to a second voltage input, and a second drain coupled to thefirst drain. A second pull-up transistor is formed in the secondsemiconductor structure and comprises a third gate, a third source and athird drain with the third source being coupled to the secondsemiconductor structure such that the third source is coupled to thefirst voltage input through parasitic resistance of the secondsemiconductor structure. A second pull-down transistor is formed in theat least one semiconductor layer and comprises a fourth gate coupled tothe third gate, a fourth source coupled to the second voltage input, anda fourth drain coupled to the third drain. A first access transistor isformed in the at least one semiconductor layer and includes a firstterminal coupled to the first and second drains, a second terminalcoupled to a first column line and a first access gate coupled to a rowline. A second access transistor is formed in the at least onesemiconductor layer and includes a third terminal coupled to the thirdand fourth drains, a fourth terminal coupled to a second column line anda first access gate coupled to a row line. The memory array includes amemory decoder coupled to the plurality of memory cells for accessingeach of the plurality of memory cells via respective ones of a pluralityof the row lines and respective ones of a plurality of the first andsecond column lines. The microprocessor communicates with each of theplurality of memory cells via the memory decoder.

According to a still further aspect of the present invention, a methodof fabricating an SRAM memory cell is provided. A substrate assemblyhaving at least one semiconductor layer is provided. A firstsemiconductor structure is formed within the at least one semiconductorlayer. A first source and a first drain of a first pull-up transistorare formed in the first semiconductor structure. A second source and asecond drain of a first pull-down transistor are formed in the at leastone semiconductor layer. A first contact and a second contact are formedwithin the first semiconductor structure. A first gate for the firstpull-up transistor and a second gate for the first pull-down transistorare formed. The first drain is coupled to the second drain and the firstgate is coupled to the second gate. The first source is coupled to oneof the first and second contacts such that with the other of the firstand second contacts coupled to a first voltage input the first source iscoupled to the first voltage input through parasitic resistance of thefirst semiconductor structure.

According to another aspect of the present invention, a method offabricating an SRAM memory cell is provided. A substrate assembly havingat least one semiconductor layer is provided. A first semiconductorstructure and a second semiconductor structure are formed within the atleast one semiconductor layer. A first source and a first drain of afirst pull-up transistor are formed in the first semiconductorstructure. A second source and a second drain of a first pull-downtransistor are formed in the at least one semiconductor layer. A thirdsource and a third drain of a second pull-up transistor are formed inthe second semiconductor structure. A fourth source and a fourth drainof a second pull-down transistor are formed in the at least onesemiconductor layer. A first contact and a second contact are formedwithin the first semiconductor structure. A third contact and fourthcontact are formed within the second semiconductor structure. A firstgate for the first pull-up transistor, a second gate for the firstpull-down transistor, a third gate for the second pull-up transistor anda fourth gate for the second pull-down transistor are formed. The firstdrain is coupled to the second drain and the third drain is coupled tothe fourth drain. The first gate is coupled to the second gate and thethird gate is coupled to the fourth gate. The first source is coupled toone of the first and second contacts such that with the other of thefirst and second contacts coupled to a first voltage input the firstsource is coupled to the first voltage input through parasiticresistance of the first semiconductor structure. The third source iscoupled to one of the third and fourth contacts such that with the otherof the third and fourth contacts coupled to the first voltage input thethird source is coupled to the first voltage input through parasiticresistance of the second semiconductor structure.

Preferably, the first substrate and the second substrate form portionsof a single substrate. The at least one semiconductor layer may compriseP-type semiconductor material while the first semiconductor structuremay comprise an N-type well.

According to yet another aspect of the present invention, a method offabricating an SRAM memory array is provided. A substrate assemblyhaving at least one semiconductor layer is provided. A plurality ofmemory cells arranged in rows and columns are formed. Each of theplurality of memory cells are fabricated according to the followingsteps. A first semiconductor structure is formed within the at least onesemiconductor layer. A second semiconductor structure is formed withinthe at least one semiconductor layer. A first source and a first drainof a first pull-up transistor are formed in the first semiconductorstructure. A second source and a second drain of a first pull-downtransistor are formed in the at least one semiconductor layer. A thirdsource and a third drain of a second pull-up transistor are formed inthe second semiconductor structure. A fourth source and a fourth drainof a second pull-down transistor are formed in the at least onesemiconductor layer. A first terminal and a second terminal of a firstaccess transistor are formed in the at least one semiconductor layer. Athird terminal and a fourth terminal of a second access transistor areformed in the at least one semiconductor layer. A first contact and asecond contact are formed within the first semiconductor structure. Athird contact and fourth contact are formed within the secondsemiconductor structure. A first gate for the first pull-up transistor,a second gate for the first pull-down transistor, a third gate for thesecond pull-up transistor, a fourth gate for the second pull-downtransistor, a first access gate for the first access transistor, and asecond access gate for the second access transistor are formed. Thefirst drain is coupled to the second drain and the third drain iscoupled to the fourth drain. The first gate is coupled to the secondgate and the third gate is coupled to the fourth gate. The firstterminal is coupled to the first and second drains and the thirdterminal is coupled to the third and fourth drains. The first source iscoupled to one of the first and second contacts such that with the otherof the first and second contacts coupled to a first voltage input thefirst source is coupled to the first voltage input through parasiticresistance of the first semiconductor structure. The third source iscoupled to one of the third and fourth contacts such that with the otherof the third and fourth contacts coupled to the first voltage input thethird source is coupled to the first voltage input through parasiticresistance of the second semiconductor structure. The first and secondaccess gates of each of the plurality of memory cells are coupled torespective row lines. The second terminals of each of the plurality ofmemory cells are coupled to respective first column lines. The fourthterminals of each of the plurality of memory cells are coupled torespective second column lines.

Preferably, the first substrate and the second substrate form portionsof a single substrate. Each of the first and second pull-up transistorsmaking up each of the rows of memory cells may share the N-type well.

According to a still further aspect of the present invention, a methodof fabricating a computer system is provided. A memory array and amicroprocessor are provided. The memory array comprises a plurality ofmemory cells arranged in rows and columns and formed on a substrateassembly comprising at least one semiconductor layer. Each of theplurality of memory cells comprise a first semiconductor structure and asecond semiconductor structure formed within the at least onesemiconductor layer with the first and second semiconductor structuresis coupled to a first voltage input. A first pull-up transistor isformed in the first semiconductor structure and comprises a first gate,a first source and a first drain with the first source being coupled tothe first semiconductor structure such that the first source is coupledto the first voltage input through parasitic resistance of the firstsemiconductor structure. A first pull-down transistor is formed in theat least one semiconductor layer and comprises a second gate coupled tothe first gate, a second source coupled to a second voltage input, and asecond drain coupled to the first drain. A second pull-up transistor isformed in the second semiconductor structure and comprises a third gate,a third source and a third drain with the third source being coupled tothe second semiconductor structure such that the third source is coupledto the first voltage input through parasitic resistance of the secondsemiconductor structure. A second pull-down transistor is formed in theat least one semiconductor layer and comprises a fourth gate coupled tothe third gate, a fourth source coupled to the second voltage input, anda fourth drain coupled to the third drain. A first access transistor isformed in the at least one semiconductor layer and includes a firstterminal coupled to the first and second drains, a second terminalcoupled to a first column line and a first access gate coupled to a rowline. A second access transistor is formed in the at least onesemiconductor layer and includes a third terminal coupled to the thirdand fourth drains, a fourth terminal coupled to a second column line anda first access gate coupled to a row line. The memory array includes amemory decoder coupled to the plurality of memory cells for accessingeach of the plurality of memory cells via respective ones of a pluralityof the row lines and respective ones of a plurality of the first andsecond column lines. The microprocessor communicates with each of theplurality of memory cells via the memory decoder.

Accordingly, it is an object of the present invention to provide animproved SRAM that is not prone to latch up; and, to provide an improvedSRAM memory cell wherein parasitic resistance is used to couple V_(CC)to pull-up transistors to prevent latch-up of the memory cell. Otherfeatures and advantages of the invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art six transistor SRAM memory cell;

FIG. 2A illustrates one of the inverters of the SRAM memory cell of FIG.1 along with the corresponding parasitic transistors and resistors;

FIG. 2B is an enlarged, sectioned side view depicting the inverter andparasitic transistors and resistors of FIG. 2A;

FIG. 3 illustrates schematically an SRAM memory cell in accordance withthe present invention;

FIG. 4A illustrates one of the inverters of the SRAM memory cell of FIG.3 along with the corresponding parasitic transistors and resistors;

FIG. 4B is an enlarged, sectional side view depicting the inverter andparasitic transistors and resistors of FIG. 4A;

FIG. 5 is an enlarged, sectional side view of the SRAM memory cell ofFIG. 3 according to one aspect of the present invention;

FIG. 6 illustrates schematically an SRAM memory array using the SRAMmemory cell of FIG. 3;

FIG. 7 is a plan view of the layout of the SRAM memory array of FIG. 6according to another aspect of the present invention; and

FIG. 8 illustrates schematically a computer system using the SRAM memoryarray of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 illustrates schematically an SRAM memory cell 30 in accordancewith the present invention. The SRAM memory cell 30 includes a first MOS(p-type) pull-up transistor 32, a first MOS (n-type) pull-downtransistor 34, a second MOS (p-type) pull-up transistor 36, a second MOS(n-type) pull-down transistor 38, a third MOS or first access transistor40 and a fourth MOS or second access transistor 42. The MOS transistors32, 34, 36 and 38 each include a drain (D), a gate (G) and a source (S)as illustrated in FIG. 3 with the first pull-up transistor 32 having afirst drain 32D, a first source 32S and a first gate 32G, the firstpull-down transistor 34 having a second drain 34D, a second source 34Sand a second gate 34G, the second pull-up transistor 36 having a thirddrain 36D, a third source 36S and a third gate 36G and the secondpull-down transistor 38 having a fourth drain 38D, a fourth source 38Sand a fourth gate 38G, see FIGS. 4A, 4B and 5.

It will be appreciated by those skilled in the art that the drain andsource terminals of a MOS transistor are typically identical with thedrain/source label being applied for descriptive purposes once a voltageis applied to the transistor. For n-type transistors, the draindesignation is applied to the terminal having the higher voltagepotential with the source designation being applied to the otherterminal. Accordingly, separate drain/source designations have not beenapplied to the access transistors 40 and 42 as voltages across thedrain/source terminals change in such a manner as to cause correspondingchanges in drain/source designations for the access transistors 40 and42. The drains and sources of the first and second access transistors 40and 42 will be described as first terminals 40A, 42A and secondterminals 40B, 42B for descriptive purposes herein.

Each of the MOS transistors 32, 34, 36 and 38 have substrate terminalsSUB corresponding to the semiconductor substrates in which thetransistors are formed. The substrates of the pull-up transistors 32 and36 are coupled to a first voltage input V_(CC). The sources 32S, 36S ofthe pull-up transistors 32 and 36 are coupled to their correspondingsubstrates such that the sources are coupled to the first voltage inputV_(CC) through the parasitic resistance of each substrate represented byresistors 44 and 46. The drains 32D, 36D of the pull-up transistors 32and 36 are coupled to a first node 48 and a second node 50,respectively. The drains 34D, 38D of the pull-down transistors 34 and 38are coupled to the first node 48 and the second node 50, respectively.The sources 34S, 38S of the pull-down transistors 34 and 38 are coupledto a second voltage input or V_(SS). Typically, V_(CC) is approximately5.0 volts to 2.0 volts, depending on the process and technology, whileV_(SS) is approximately zero volts or ground.

The transistors 32, 34, 36 and 38 form a pair of cross-coupled CMOSinverters with transistors 32 and 34 configured as a first CMOS inverter35 and transistors 36 and 38 configured as a second CMOS inverter 39.The pull-down transistors 34 and 38 are cross-coupled with the drain 34Dof the first pull-down transistor 34 being coupled to the gate 38G ofthe second pull-down transistor 38 and the drain 38D of the secondpull-down transistor 38 being coupled to the gate 34G of the firstpull-down transistor 34. For illustrative purposes, it is assumed thatlogic state “0” is designated by node 48 having a low voltage and node50 having a high voltage while logic state “1” is designated by node 48having a high voltage and node 50 having a low voltage.

In logic state “0” the high voltage on node 50 turns on the firstpull-down transistor 34 and turns off the first pull-up transistor 32,whereas the low voltage on node 48 turns off the second pull-downtransistor 38 and turns on the second pull-up transistor 36. Because thefirst pull-down transistor 34 is on and the first pull-up transistor 32is off, current flows through the first pull-down transistor 34 to thesecond voltage input V_(SS), thereby maintaining a low voltage on node48. Because the second pull-up transistor 36 is turned on and the secondpull-down transistor 38 is turned off, current flows from the firstvoltage input V_(CC) through the second pull-up transistor 36, therebymaintaining a high voltage on node 50. It should be apparent that forlogic state “1,” the on/off states of the transistors are reversed withcorresponding current flows.

The first terminal 40A of the first access transistor 40 is coupled tothe first node 48 while the second terminal 40B is coupled to a firstbit or column line 52. The first access gate 40G of the first accesstransistor 40 is coupled to a word or row line 54. Similarly, the firstterminal 42A of the second access transistor 42 is coupled to the secondnode 50 while the second terminal 42B is coupled to a second bit orcolumn line 56. The second access gate 42G of the second accesstransistor 42 is coupled to the word or row line 54. Typically, thefirst and second column lines 52, 56 receive symmetrical data pulseswith one of the column lines receiving the complement of the othercolumn line. It should be apparent that the first and second accesstransistors 40 and 42 function as an input line for providing access tothe memory cell 30.

To change the state of the SRAM cell 30 from a logic “0” to a logic “1”,the second column line 56 and the first column line 52 are provided witha low and a high voltage, respectively. Then, the access transistors 40and 42 are turned on by a high voltage on the row line 54, therebyproviding the low voltage on the second column line 56 to node 50 andthe high voltage on the first column line 52 to node 48. Accordingly,the first pull-down transistor 34 is turned off and the first pull-uptransistor 32 is turned on by the low voltage on node 50, and the secondpull-down transistor 38 is turned on and the second pull-up transistor36 is turned off by the high voltage on node 48, thereby switching thestate of the circuit from logic “0” to logic “1”. Following theswitching of the state of the SRAM cell 30, the access transistors 40and 42 are turned off (by applying a low voltage on row line 54). TheSRAM cell 30 maintains its new logic state in a manner analogous to thatdescribed above.

Referring now to FIGS. 4A and 4B, the effect of the parasitic devicesresulting from the configuration of the SRAM cell 30 will now bedescribed. A schematic diagram of the first CMOS inverter 35 isillustrated in FIG. 4A with its corresponding parasitic transistors andresistors. It will be appreciated by those skilled in the art that thesecond inverter 39 includes similar parasitic transistors and resistors.A cross-section of the first CMOS inverter 35 of FIG. 4A is illustratedin FIG. 4B. As shown in FIG. 4B, the first inverter 35, and hence, thememory cell 30, is formed on a substrate assembly 60 comprising asemiconductor layer 62 that is silicon in the illustrated embodiment,and may also include additional layers or structures that define activeor operable portions of semiconductor devices (not shown). For example,the semiconductor layer 62 of the substrate assembly 60 may be formed oninsulating material, sapphire or another base material.

The semiconductor layer 62 is doped with impurities to form asemiconductor of a first or p-type conductivity. A first semiconductorstructure formed within the semiconductor layer 62 is preferably formedof a second or n-type conductivity commonly referred to as an N-well 64.The first pull-down transistor 34 is formed within the semiconductorlayer 62 and the first pull-up transistor 32 is formed within the N-well64.

Buried contacts are formed within the semiconductor layer 62 and theN-well 64 to form the second source 34S and the second drain 34D of thefirst pull-down transistor 34, and the first source 32S and the firstdrain 32D of the first pull-up transistor 32. A first contact 66 and asecond contact 68 are also formed within the N-well 64 while a firsttie-down contact 70 is formed in the semiconductor layer 62. The firstgate 32G of the first pull-up transistor 32 and the second gate 34G ofthe first pull-down transistor 34 are also formed. It should be apparentthat the N-well 64 and the semiconductor layer 62 represent thesemiconductor substrates in which the transistors 32 and 34 are formed.The first contact 66 and the first tie-down contact 70 are thus used tobias the N-well/first pull-up transistor substrate 64 and thesemiconductor layer/first pull-down transistor substrate 62,respectively. It will be appreciated by those skilled in the art thatthe N-well 64, the buried contacts, and the gates may be formed usingprocesses well known in the art.

Appropriate electrically conductive interconnect layers are formed usingprocesses well known in the art to couple the first gate 32G to thesecond gate 34G, the first drain 32D to the second drain 34D, and thefirst source 32S to the second contact 68. Additional electricallyconductive interconnect layers are formed to couple the first contact 66to the first voltage input V_(CC), the second source 34S to the secondvoltage input V_(SS), and the first tie-down contact 70 to the substrate62 tie down V_(BB). The N-well 64 includes parasitic resistance denotedby the resistors 44A and 44B while the substrate 62 includes parasiticresistance denoted by the resistor 72. The configuration of the firstinverter 35 also results in the existence of a PNP parasitic bipolartransistor 74 and an NPN parasitic bipolar transistor 76.

The parasitic resistor 44 is shown as two resistors 44A and 44B todenote the parasitic resistance between the second contact 68 and thebase of the parasitic PNP bipolar transistor 74 and the parasiticresistance between the base of the parasitic PNP bipolar transistor 74and the first contact 66. The first source 32S is thus coupled to thefirst voltage input V_(CC) through the combined parasitic resistance ofthe N-well 64 represented by the parasitic resistors 44A and 44B. Thesubstrate connection of the first pull-up transistor 32 is shown as avariable resistor in FIG. 4A because the resultant parasitic resistanceof the N-well 64 varies depending on the distance between the firstcontact 66 and the first source 32S, i.e., the further the separationthe greater the resistance.

As is stated earlier and clearly shown in FIG. 4A and FIG. 4B, theparasitic resistance of the N-well 64 is represented by the parasiticresistor 44 and varies as a function of the separation between the firstcontact 66 and the second contact 68. The parasitic resistance of theN-well 64 is a combination of a first component and a second component.The first and second components are represented by parasitic resistors44A and 44 B, respectively. The relative separations of the firstcontact 66 and the first source 32S and the second contact 68 and thefirst source 32S of FIG. 4B, show that the first component of theparasitic resistance is at least as large as the second component of theparasitic resistance.

Leakage current from the N-well 64 produces a voltage drop across theparasitic resistor 44A. However, with the first source 32S coupled tothe first voltage input V_(CC) through the N-well 64, the first source32S is always at a potential less than or equal to the potential of theN-well 64. The net result of this configuration is that the emitter-basejunction of the parasitic PNP transistor 74 cannot become forward biasedand latch-up cannot occur.

Referring now to FIG. 5, the cross-sectional layout of the SRAM memorycell 30 is shown without the parasitic resistors and transistors. Thesecond pull-up transistor 36 is formed within a second semiconductorstructure or N-well 78 while the second pull-down transistor 38 isformed within the semiconductor layer 62. Buried contacts are formedwithin the semiconductor layer 62 and the N-well 78 to form the fourthsource 38S and the fourth drain 38D of the second pull-down transistor38, and the third source 36S and the third drain 36D of the secondpull-up transistor 36 using processes well known in the art. A thirdcontact 80 and a fourth contact 82 are also formed within the N-well 78while a second tie-down contact 84 is formed in the semiconductor layer62 using processes well known in the art. The third gate 36G of thesecond pull-up transistor 36 and the fourth gate 38G of the secondpull-down transistor 38 are formed using processes well known in theart. It should be apparent that the N-well 78 and the semiconductorlayer 62 represent the semiconductor substrates in which the transistors36 and 38 are formed, respectively. The third contact 80 and the secondtie-down contact 84 are thus used to bias the N-well/second pull-uptransistor substrate 78 and the semiconductor layer/first pull-downtransistor substrate 62, respectively.

Appropriate electrically conductive interconnect layers are formed usingprocesses well known in the art to couple the third gate 36G and thefourth gate 38G to the first drain 32D and the second drain 34D, thethird drain 36D and the fourth drain 38D to the first gate 32G and thesecond gate 34G, and the third source 36S to the fourth contact 82.Additional electrically conductive interconnect layers are formed tocoupled the third contact 80 to the first voltage input V_(CC), thefourth source 38S to the second voltage input V_(SS) and the secondtie-down contact 84 to the substrate 62 tie down V_(BB). It will beappreciated by those skilled in the art that the first and secondpull-down transistors 34 and 38 may be arranged so that only onetie-down contact is needed for the memory cell 30. It will be furtherappreciated by those skilled in the art that a single N-well may be usedto form the first and second pull-up transistors 32 and 36. It will beeven further appreciated by those skilled in the art that the first andsecond pull-up transistors 32 and 36 may be configured to share the samesource contact. Accordingly, with the first semiconductor structure orN-well 64 the same as the second semiconductor structure or N-well 78,i.e., forming portions of a single substrate, only one contact would beneeded to couple the sources to the N-well and only one contact would beneeded to couple the N-well to the first voltage input V_(CC).

Referring now to FIG. 6, it is contemplated by the present inventionthat the SRAM memory cell 30, described above with respect to FIG. 3,may be utilized to provide an SRAM memory array 90. The SRAM memoryarray 90 comprises a plurality of SRAM memory cells 30 arranged in adesired number of rows and columns. FIG. 6 depicts an illustrative 16cell memory array having four (4) rows and four (4) columns. Each of thecolumns include respective first and second column lines 52 ₁–52 ₄, 56₁–56 ₄ while each of the rows include respective row lines 54 ₁–54 ₄.The column and row lines 52 ₁–52 ₄, 56 ₁–56 ₄ and 54 ₁–54 ₄ are coupledto a memory decoder 92. The memory decoder 92 is capable of assessingeach of the memory cells 30 through a unique memory command conveyed onthe column and row lines 52 ₁–52 ₄, 56 ₁–56 ₄ and 54 ₁–54 ₄.

The first and second pull-up transistors 32 and 36 of the SRAM memorycells 30 making up each row of the memory array 90 are preferably formedwithin the same N-well as shown in FIG. 7. Four N-wells 64 ₁–64 ₄ witheach of the first and second pull-up transistors 32 and 36 for eachmemory cell 30 in each row sharing a respective N-well are shown in FIG.7. For illustrative purposes, the two horizontal boxes of each cellrepresent the gate connections for each pull-up transistor 32, 36 whilethe vertical box represents the active areas for the transistors 32, 36.It will be appreciated by those skilled in the art that the first andsecond pull-down transistors 34, 38 for each memory cell 30 would beappropriately formed on both sides of the respective N-wells 64 ₁–64 ₄.

Each of the sources 32S, 36S are coupled to respective N-wellmetalization lines 94 ₁–94 ₄ with the N-well metalization lines 94 ₁–94₄ being coupled to at least one V_(CC) metalization line 96 receivingthe first voltage input V_(CC). Further, each row includes at least onecontact 68 ₁–68 ₄ for coupling each respective N-well 64 ₁–64 ₄ to therespective N-well metalization line 94 ₁–94 ₄, and hence, to the firstvoltage input V_(CC) through the V_(CC) metalization line 96. In theillustrated embodiment, there is one contact 68 for every four memorycells 30. However, it will be appreciated by those skilled in the artthat this four to one ratio may be increased or decreased as appropriatefor each particular application. For example, one contact 68 may besufficient to bias the N-well 64 for every 32 memory cells 30. It willbe further appreciated by those skilled in the art that additionalV_(CC) metalization lines 96 may be added as appropriate.

Referring now to FIG. 8, it is contemplated by the present inventionthat the SRAM memory array 90, described above with respect to FIG. 6,may be utilized to provide an SRAM memory array 90 within a computersystem 100. As will be appreciated by those skilled in the art, thecomputer system 100 would include, as appropriate, a ROM 102, a massmemory 104, peripheral devices 106, and I/O devices 108 in communicationwith a microprocessor 110 via a data bus 112 or another suitable datacommunication path. The microprocessor 110 communicates with each of theplurality of memory cells 30 via the memory decoder 92.

Having described the invention in detail and by reference to preferredembodiments thereof, it will be apparent that modifications andvariations are possible without departing from the scope of theinvention defined in the appended claims.

1. A method of fabricating a memory cell comprising the steps of:providing a substrate assembly having at least one semiconductor layer;forming a first semiconductor structure within said at least onesemiconductor layer; forming a first source and a first drain of a firstpull-up transistor in said first semiconductor structure; forming asecond source and a second drain of a first pull-down transistor in saidat least one semiconductor layer; forming a first contact and a secondcontact within said first semiconductor structure; forming a first gatefor said first pull-up transistor and a second gate for said firstpull-down transistor; coupling said first drain to said second drain;coupling said first gate to said second gate; and coupling said firstsource to one of said first and second contacts such that with the otherof said first and second contacts coupled to a first voltage input saidfirst source is coupled to said first voltage input through parasiticresistance of said first semiconductor structure.
 2. The method of claim1, further comprising the step of forming an access transistor in saidat least one substrate assembly, said access transistor having oneterminal coupled to said first and second drains.
 3. The method of claim2, wherein said access transistor further comprises an additionalterminal and an access gate, said additional terminal coupled to acolumn line of a memory array and said access gate coupled to a row lineof said memory array.
 4. The method of claim 1, wherein: said firstcontact is formed a first distance from said first source; said firstdistance defines a first component of said parasitic resistance; saidsecond contact is formed a second distance from said first source; andsaid second distance defines a second component of said parasiticresistance.
 5. The method of claim 4, further comprising the step offorming a first parasitic bi-polar transistor including a base, whereinsaid base is coupled to said first contact through said second componentof said parasitic resistance and to said second contact through saidfirst component of said parasitic resistance.
 6. The method of claim 4,wherein said first component of said parasitic resistance and saidsecond component of said parasitic resistance are connected in series.7. The method of claim 4, wherein said first component of said parasiticresistance is at least as large as said second component of saidparasitic resistance.
 8. The method of claim 4, wherein said firstcomponent of said parasitic resistance is larger than said secondcomponent of said parasitic resistance.
 9. The method of claim 5,wherein a base emitter junction of said first parasitic bipolartransistor is configured such that the junction can not become forwardbiased.
 10. The method of claim 1, wherein a resistance value of saidparasitic resistance is as a function of the separation between saidfirst and second contacts.
 11. The method of claim 1, wherein: said atleast one semiconductor layer comprises P-type semiconductor material;and, wherein said semiconductor structure comprises an N-type well. 12.The method of claim 1, wherein said first pull-up transistor and saidfirst pull-down transistor form an inverter.
 13. The method of claim 12,wherein said inverter is a CMOS inverter.
 14. The method of claim 1,wherein said semiconductor structure is configured to define an SRAMmemory cell.
 15. The method of claim 1, wherein said first source isalways at a potential less than or equal to a potential of said firstvoltage input.
 16. The method of claim 1, wherein said first source iscoupled to one of said first and second contacts by forming anelectrically conducting interconnect layer over said first semiconductorstructure.
 17. The method of claim 15, wherein said conductinginterconnect layer comprises a metallization connection.
 18. The methodof claim 1, wherein said first pull-up transistor is formed between saidfirst and second contacts.
 19. A method of fabricating a memory cellcomprising the steps of: providing a substrate assembly having at leastone semiconductor layer; forming a first semiconductor structure and asecond semiconductor structure within said at least one semiconductorlayer; forming a first source and a first drain of a first pull-uptransistor in said first semiconductor structure; forming a secondsource and a second drain of a first pull-down transistor in said atleast one semiconductor layer; forming a third source and a third drainof a second pull-tip transistor in said second semiconductor structure;forming a fourth source and a fourth drain of a second pull-downtransistor in said at least one semiconductor layer; forming a firstcontact and a second contact within said first semiconductor structure;forming a third contact and fourth contact within said secondsemiconductor structure; forming a first gate for said first pull-uptransistor, a second gate for said first pull-down transistor, a thirdgate for said second pull-up transistor and a fourth gate for saidsecond pull-down transistor; coupling said first drain to said seconddrain and said third drain to said fourth drain; coupling said firstgate to said second gate and said third gate to said fourth gate; andcoupling said first source to one of said first and second contacts suchthat with the other of said first and second contacts coupled to a firstvoltage input said first source is coupled to said first voltage inputthrough parasitic resistance of said first semiconductor structure; andcoupling said third source to one of said third and fourth contacts suchthat with the other of said third and fourth contacts coupled to saidfirst voltage input said third source is coupled to said first voltageinput through parasitic resistance of said second semiconductorstructure.
 20. The method of claim 19, wherein said first semiconductorstructure and said second semiconductor structure form portions of asingle substrate.